In conventional large houses, two-car garages are often used. Such large garages require corresponding large doors, which are usually made of a number of superimposed rectangular wooden panels, hingedly connected in pairs about a common plane. The door is supported and guided by rails, each having a horizontal section anchored to the ceiling of the garage. When the door is closed, all these wooden panels are vertical. When the door is to be opened, it is pulled upwardly along its rails so that the uppermost panel, and subsequently all the other successively lower panels, be pivoted for 90.degree. from a vertical position to a horizontal position, about their interconnecting hinges. Therefore, the panels must be sufficiently strong to avoid bending when in horizontal position.
It can be readily understood that these garage doors need to be thermally insulated during winter in cold climates. Conventional garage door panels are made of metal and are hollow, and are either injected with urethane foam (the best method, although it is expensive) or a rigid styrofoam sheet is inserted into the hollow. The latter method is cheap, but with time and wear, the insulating sheet will shrink, as is well known in the art, whereby cold exterior air will seep through the door panels and the insulation will be ineffective. It has also been found that warping of the panels occurs due to exposure to large temperature differentials. Patio doors and large windows of conventional construction have metallic frames for rigidity but these frames constitute a thermal bridge.